As large metallic structures, such as, for example, aircraft, age, significant damage to their corrosion protective systems may occur. These corrosion protective systems typically include various sealants, primers and paint. For examples paint on the structure may scratch and crack around fasteners and other joints, sealants may become disbonded from the underlying substrate, and tiny cracks may develop in the coatings through ordinary use. When such damage occurs, water, salt, and other contaminants may intrude through the corrosion protective system and initiate corrosion in the underlying metallic structure.
In general, corrosion occurs by oxidation of the metal in an anodic area with concurrent reduction, typically of hydrogen, in the cathodic region. When corrosion initially develops, it is generally difficult to detect under the paint, primer and/or sealant that may be applied to the surface of the metallic structure. As the metallic structure ages, the potential for failure resulting from stress corrosion induced cracking, disbanding of stiffeners, and structural weakening due to loss of material increases. Having the ability to detect corrosion early in its development would greatly reduce repair costs and, in some instances where the metallic structure is used in, for example, an aircraft, a pressure vessel, a ship, or any other aluminum structure, increase safety. Corrosion on such structures can cause the structures to be life-limited or require patches or fittings to strengthen an area that has had deep corrosion removed. These repairs, which often require structural analysis, are especially costly. Early detection of the corrosion would eliminate the need for such repairs or allow for early and inexpensive repairs to be made.
Several prior art methods exist for detection of corrosion on metallic structures. Such methods include visual inspections, fluorescent penetrant, eddy current, ultrasonics, and radiography. Visual inspection methods (even if aided by a magnifier or borescope) and fluorescent penetrant methods typically only permit detection of corrosion on the outer layer of the structure and do not have the ability to quantify the amount of corrosion. The use of paint, primer and sealants on the structures greatly decreases the effectiveness or usefulness of these two detection methods as generally the coatings must be removed from the structure prior to using either of these two methods. Additionally, visual inspection techniques generally require highly trained personnel with field experience in detecting and identifying corrosion.
Ultrasound, eddy current and radiography techniques are only sensitive to material thinning, not the presence of corrosion. Significant material loss, for examples greater than 5%, must occur before eddy current or ultrasonic techniques can reliably detect material loss due to corrosion. While radiography has proven to be effective in detection of hidden corrosion in certain structures, a quantitative assessment of material loss due to corrosion cannot be made. Radiography has the additional disadvantages of high cost, safety concerns, and lack of portability.
These prior art techniques suffer from significant disadvantages in addition to those previously mentioned. For example, they are severely limited for use in detecting hidden corrosion in inaccessible areas.
Accordingly, there is a continuing need for a nondestructive evaluation method that can be used to detect and characterize hidden corrosion within a complex metallic structure such as an aircraft. Such a method would desirably detect corrosion hidden underneath non-ferromagnetic paint, primer, sealants, polymers, and fillers; be sensitive to water ingress; and demonstrate sensitivity to nascent or incipient corrosion on aluminum and aluminum alloys. In addition, the method would quantify the amount of corrosion present in the structure.